Amplifier class AB output stage

ABSTRACT

An amplifier includes an input stage, a folded cascode stage, and a class AB output stage. The folded cascode stage is coupled to the input stage. The class AB output stage is coupled to the folded cascode stage. The class AB output stage includes a high-side output transistor, a low-side output transistor, and a high-side feedback circuit that is coupled to the high-side output transistor. The high-side feedback circuit includes a high-side sense transistor and a high-side feedback transistor. The high-side sense transistor includes a control terminal that is coupled to a control terminal of the high-side output transistor. The high-side feedback transistor is coupled to an output of the high-side sense transistor and to the folded cascode stage. A first output of the folded cascode stage is coupled to the control terminal of the high-side sense transistor and to the control terminal of the high-side output transistor.

CROSS REFERENCE TO RELATED APPLICATION(S)

This continuation application claims priority to U.S. patent applicationSer. No. 15/912,642, filed Mar. 6, 2018, which application isincorporated herein by reference in its entirety.

BACKGROUND

Of the many available electronic devices, operational amplifiers(op-amps) are some of the most widely used. Op-amps are efficient andversatile devices that can be used in a variety of applications, such assignal conditioning, analog instrumentation, analog computation, etc.

An op-amp may employ one of several different circuit arrangements. Inone example, a class A amplifier reproduces an entire input signalbecause an active element of the class A amplifier, such as atransistor, is constantly in the active mode. Class A amplifierstypically have high power consumption because the active elementconstantly conducts current.

In another example, a class B amplifier typically employs twocomplementary output transistors, with each output transistor beingturned on for half of the time and turned off for the other half of thetime. That is, one output transistor operates as a current source, andthe other output transistor operates as a current sink. Thisconfiguration is sometimes referred to as a “push-pull” configurationsince a first branch of the output stage “pushes” or sources currents toa load while a second branch of the output stage “pulls” or sinkscurrent from the load. A class B amplifier has lower power consumptionthan a class A amplifier but may be susceptible to crossover distortiondue to the turn-on of one output transistor not matching the turn-off ofthe other output transistor, and may be unstable with capacitive loadsat low currents.

A class AB amplifier also employs two complementary output transistors(similar to a class B amplifier). Class AB amplifiers avoid the highpower consumption of a class A amplifier by always having one outputbranch substantially turn off when the other output branch is turned on.Although the current in one leg of a class AB amplifier is substantiallyturned off there is a small amount of current flowing in that leg. Thesmall residual current in the class AB amplifier avoids the crossoverdistortion produced by the turning on and off of the currents in class Bamplifiers. Thus, class AB amplifiers are able to achieve a relativelyhigh current output while maintaining a low quiescent current andproviding stability with capacitive loads at any load current.

SUMMARY

A class AB amplifier that provides high gain at low power supplyvoltages is disclosed herein. In one example, an amplifier includes aninput stage, a folded cascode stage, and a class AB output stage. Thefolded cascode stage is coupled to the input stage. The class AB outputstage is coupled to the folded cascode stage. The class AB output stageincludes a high-side output transistor, a low-side output transistor,and a high-side feedback circuit. The high-side feedback circuit iscoupled to the high-side output transistor. The high-side feedbackcircuit includes a high-side sense transistor and a high-side feedbacktransistor. The high-side sense transistor includes a control terminalthat is coupled to a control terminal of the high-side outputtransistor. The high-side feedback transistor is coupled to an output ofthe high-side sense transistor and to the folded cascode stage. A firstoutput of the folded cascode stage is coupled to the control terminal ofthe high-side sense transistor and to the control terminal of thehigh-side output transistor.

In another example, a class AB amplifier includes a folded cascode stageand a class AB output stage. The class AB output stage is coupled to thefolded cascode stage. The class AB output stage includes a high-sideoutput transistor, a low-side output transistor, a high-side feedbackcircuit, and a low-side feedback circuit. The high-side feedback circuitis coupled to the high-side output transistor. The high-side feedbackcircuit includes a high-side sense transistor and a first currentsource. The high-side sense transistor includes a control terminal andan output terminal. The control terminal is coupled to a controlterminal of the high-side output transistor. The output terminal iscoupled to the folded cascode stage. The first current source is coupledto an output of the high-side sense transistor and to a first referencevoltage source. A first output of the folded cascode stage is coupled tothe control terminal of the high-side sense transistor and to thecontrol terminal of the high-side output transistor. The low-sidefeedback circuit is coupled to the low-side output transistor. Thelow-side feedback circuit includes a low-side sense transistor and asecond current source. The low-side sense transistor includes a controlterminal and an output terminal. The control terminal is coupled to acontrol terminal of the low-side output transistor. The output terminalis coupled to the folded cascode stage. The second current source iscoupled to an input of the low-side sense transistor and to a secondreference voltage source. A second output of the folded cascode stage iscoupled to the control terminal of the low-side sense transistor and tothe control terminal of the low-side output transistor.

In a further example, a class AB amplifier includes a high-side outputtransistor, a low-side output transistor, a folded cascode stage, ahigh-side feedback circuit, and a low-side feedback circuit. The foldedcascode stage is coupled to the high-side output transistor and thelow-side output transistor. The high-side feedback circuit is coupled tothe high-side output transistor. The high-side feedback circuit includesa high-side sense transistor and a high-side feedback transistor. Thehigh-side sense transistor includes a control terminal and an outputterminal. The control terminal is coupled to a control terminal of thehigh-side output transistor. The output terminal is coupled to thefolded cascode stage. The high-side feedback transistor is coupled to anoutput of the high-side sense transistor and to the folded cascodestage. A first output of the folded cascode stage is coupled to thecontrol terminal of the high-side sense transistor and to the controlterminal of the high-side output transistor. The low-side feedbackcircuit is coupled to the low-side output transistor. The low-sidefeedback circuit includes a low-side sense transistor and a low-sidefeedback transistor. The low-side sense transistor includes a controlterminal and an output terminal. The control terminal is coupled to acontrol terminal of the low-side output transistor. The output terminalis coupled to the folded cascode stage. The low-side feedback transistoris coupled to an output of the low-side sense transistor and to thefolded cascode stage. A second output of the folded cascode stage iscoupled to the control terminal of the low-side sense transistor and tothe control terminal of the low-side output transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of various examples, reference will now bemade to the accompanying drawings in which:

FIG. 1 shows a block diagram of an example of a class AB amplifier inaccordance with the present disclosure; and

FIG. 2 shows a schematic diagram for an example of a cascode stage and aclass AB output stage of an amplifier in accordance with the presentdisclosure.

DETAILED DESCRIPTION

Certain terms have been used throughout this description and claims torefer to particular system components. As one skilled in the art willappreciate, different parties may refer to a component by differentnames. This document does not intend to distinguish between componentsthat differ in name but not function. In this disclosure and claims, theterms “including” and “comprising” are used in an open-ended fashion,and thus should be interpreted to mean “including, but not limited to .. . .” Also, the term “couple” or “couples” is intended to mean eitheran indirect or direct wired or wireless connection. Thus, if a firstdevice couples to a second device, that connection may be through adirect connection or through an indirect connection via other devicesand connections. The recitation “based on” is intended to mean “based atleast in part on.” Therefore, if X is based on Y, X may be a function ofY and any number of other factors.

Operational amplifiers are frequently used in battery power devices orsystems in which it is desirable to minimize the energy consumed by theamplifier (e.g., to maximize the operational life of the battery).Reduction of energy consumption is often accomplished by lowering thepower supply voltages provided to the operational amplifier.

When attempting to provide low-voltage operation in a class ABamplifier, various requirements of class AB control should beconsidered. Class AB current should be accurately controlled when loadcurrent is near zero to ensure a predetermined minimum capacitive drivecapability, as the capacitive drive capability defines the minimumtransconductance of the amplifier's output transistors and the loadcaused pole g_(m)/C_(Load). Class AB control should also reduce thenonlinear distortions caused by the change in transconductance versuscurrent in the amplifier's output transistors. Maintaining a minimumtransconductance, defined by the class AB current, decreases the totaltransconductance versus load current variation. Further, class ABcontrol should prevent complete shutdown of the output transistors overthe full range of load current because shutdown can cause unpredictabledistortion and delay in operation.

Some class AB amplifiers provide class AB control using negativefeedback circuitry that maintains a minimum input current to each of theoutput transistors and maintains overall amplifier gain by shiftingcurrent from the input of a first output transistor to the input of asecond output transistor as the first output transistor is turning off.Some implementations of negative feedback circuitry for controllingclass AB operation are operable only at relatively high power supplyvoltages (e.g., 2.1-2.2 volts). Some implementations are operable atrelatively low power supply voltages (1.7-1.8 volts) but require the useof low threshold transistors, which are not available in allsemiconductor processes.

The class AB amplifiers disclosed herein include class AB controlcircuitry that is operable at low power supply voltages (e.g., 1.2-1.5volts) without use of low threshold transistors.

FIG. 1 shows a block diagram of an example of a class AB amplifier 100in accordance with the present disclosure. The class AB amplifier 100includes an input stage 102, a folded cascode stage 104, and a class ABoutput stage 106. The class AB amplifier 100 may include variouscircuits and interconnections that have been omitted in the interest ofclarity. For example, the input stage 102, the folded cascode stage 104,and the class AB output stage 106 are connected to power supplies thathave been omitted from FIG. 1.

The input stage 102 receives an input signal from source external to theclass AB amplifier 100, and applies gain to the input signal. The inputsignal received by the input stage 102 may be differential, and theinput stage 102 may include transistors arranged as a differentialamplifier. Amplified signal produced by the input stage 102 is providedto the folded cascode stage 104.

The folded cascode stage 104 receives the signal (e.g., the differentialsignal) output of the input stage 102 and applies additional gain to thereceived signal. The folded cascode stage 104 may include one or morefolded cascode circuits. Each folded cascode circuit may includetransistors arranged as current sources and transistors arranged ascascodes. Amplified signal produced by the folded cascode stage 104 isprovided to the class AB output stage 106.

The class AB output stage 106 receives the signal (e.g., thedifferential signal) output of the folded cascode stage 104, and appliesadditional gain to the received signal as needed to drive a loadexternal to the class AB amplifier 100. The class AB output stage 106includes output transistors 108 and class AB control circuitry 110. Theoutput transistors 108 include a high-side output transistor and alow-side output transistor, each of which is driven by the signal outputof the folded cascode stage 104. The class AB control circuitry 110includes feedback circuitry that controls the currents flowing in theoutput transistors 108. The feedback circuitry includes high-sidefeedback circuitry 112 and low-side feedback circuitry 114. Thehigh-side feedback circuitry 112 senses the current flowing in thehigh-side output transistor and generates a feedback signal that isprovided to the folded cascode stage 104 to adjust the output signal ofthe folded cascode stage 104 that controls the high-side outputtransistor. Similarly, the low-side feedback circuitry 114 senses thecurrent flowing in the low-side output transistor and generates afeedback signal that is provided to the folded cascode stage 104 toadjust the output signal of the folded cascode stage 104 that controlsthe low-side output transistor. The high-side feedback circuitry 112 andlow-side feedback circuitry 114 include no low voltage thresholdtransistors, and accordingly, may be fabricated without specialsemiconductor processing.

FIG. 2 shows a schematic diagram for an example of a folded cascodestage 204 and a class AB output stage 206 of an amplifier in accordancewith the present disclosure. The folded cascode stage 204 is animplementation of the folded cascode stage 104, and the class AB outputstage 206 is an implementation of the class AB output stage 106. Theclass AB output stage 206 includes class AB control circuitry 210, whichis an implementation of the class AB control circuitry 110, and includesoutput transistors 208, which is an implementation of the outputtransistors 108.

The folded cascode stage 204 includes transistor 220, transistor 222,transistor 224, transistor 226, voltage source 228, transistor 236,transistor 238, transistor 240, transistor 242, and voltage sourcevoltage source 244. The transistor 220, the transistor 222, thetransistor 224, and the transistor 226 may be p-channel metal oxidesemiconductor transistors (MOSFETs). The transistor 236, the transistor238, the transistor 240, and the transistor 242 may be n-channelMOSFETs. The transistor 220, the transistor 222, the transistor 224, thetransistor 226, and the voltage source 228 are arranged as a currentsumming circuit. The transistor 220 and the transistor 222 are currentsource transistors. The transistor 224 and the transistor 226 arecascode transistors. A control terminal (e.g., a gate terminal) of thetransistor 220 is coupled to a control terminal of the transistor 222. Acontrol terminal of the transistor 224 is coupled to a control terminalof the transistor 226. An output terminal (e.g., a drain terminal) ofthe transistor 220 is coupled to an input terminal (e.g., a sourceterminal) of the transistor 224. An output terminal of the transistor222 is coupled to an input terminal of the transistor 226. An inputterminal of the transistor 220 and an input terminal of the transistor222 are coupled to a reference voltage source 280 (e.g., a power supplyrail). The voltage source 228 is coupled to the reference voltage source280 and to the control terminal of the transistor 224 and the transistor226. The output terminal of the transistor 224 is coupled to the controlterminal of the transistor 220 and the transistor 222.

The transistor 236, the transistor 238, the transistor 240, thetransistor 242, and the voltage source 244 are also arranged as acurrent summing circuit. The transistor 240 and the transistor 242 arecurrent source transistors. The transistor 236 and the transistor 238are cascode transistors. A control terminal of the transistor 240 iscoupled to a control terminal of the transistor 242. A control terminalof the transistor 236 is coupled to a control terminal of the transistor238. An output terminal (e.g., a source terminal) of the transistor 236is coupled to an input terminal (e.g., a drain terminal) of thetransistor 240. An output terminal of the transistor 238 is coupled toan input terminal of the transistor 242. An output terminal of thetransistor 240 and an output terminal of the transistor 242 are coupledto a reference voltage source 278 (e.g., a power supply rail). Thevoltage source 244 is coupled to the reference voltage source 278 and tothe control terminal of the transistor 236 and the transistor 238. Theinput terminal of the transistor 236 is coupled to the control terminalof the transistor 240 and the transistor 242.

Signal generated in the input stage 102 may be received by the foldedcascode stage 204 at nodes 230 and 234, and/or at nodes 246 and 248 ofthe folded cascode stage 204.

The output transistors 208 include high-side output transistor 246 andlow-side output transistor 248. Current flows through the high-sideoutput transistor 246 to the output node 282 to pull the output node 282towards the voltage present on the reference voltage source 280. Currentflows through the low-side output transistor 248 from the output node282 to the reference voltage source 278 to pull the output node 282towards the voltage present on the reference voltage source 278. Thehigh-side output transistor 246 may be a p-channel MOSFET and thelow-side output transistor 248 may be an n-channel MOSFET. An inputterminal of the high-side output transistor 246 may be coupled to thereference voltage source 280, and an output terminal of the high-sideoutput transistor 246 may be coupled to the output node 282. An inputterminal of the low-side output transistor 248 may be coupled to theoutput node 282, and an output terminal of the low-side outputtransistor 248 may be coupled to the reference voltage source 278. Thecontrol terminals of the high-side output transistor 246 and thelow-side output transistor 248 are coupled to the folded cascode stage204 and to the class AB control circuitry 210. Accordingly, the foldedcascode stage 204 and the class AB control circuitry 210 provide controlsignals to the high-side output transistor 246 and the low-side outputtransistor 248 that control currents flowing in the high-side outputtransistor 246 and the low-side output transistor 248.

The class AB control circuitry 210 includes a high-side feedback circuitand a low-side feedback circuit that respectively provide feedbacksignals for controlling the high-side output transistor 246 and thelow-side output transistor 248. The high-side feedback circuit includesa high-side sense transistor 250, and a high-side feedback transistor252. The high-side sense transistor 250 may be a p-channel MOSFET, andthe high-side feedback transistor 252 may be an n-channel MOSFET. Thehigh-side sense transistor 250, the high-side feedback transistor 252,and the transistor 238 of the folded cascode stage 204 form a feedbackloop that controls the minimum current flowing in the high-side outputtransistor 246 and the high-side sense transistor 250 to be the currentprovided by the current source 274. The high-side sense transistor 250is arranged to sense the current flowing in the high-side outputtransistor 246. The control terminal of the high-side sense transistor250 is coupled to the control terminal of the high-side outputtransistor 246, and the input terminal of the high-side sense transistor250 is coupled to the reference voltage source 280. The output terminalof the high-side sense transistor 250 is coupled to the current source274 and to the control terminal of the high-side feedback transistor252. The current source 274 is coupled to the reference voltage source278.

The current flowing in the high-side sense transistor 250 isproportional to the current flowing in the high-side output transistor246. Accordingly, the voltage at the control terminal of the high-sidefeedback transistor 252 is proportional to the current flowing in thehigh-side output transistor 246, and the voltage at the control terminalof the high-side feedback transistor 252 decreases as the currentflowing in the high-side output transistor 246 decreases. The resistanceof the high-side feedback transistor 252 increases with decreasingcontrol voltage, which in-turn changes the ratio of current flowing inthe transistor 238 to the current flowing in the transistor 254 and thetransistor 256. The transistor 238 is coupled to the control terminalsof the high-side output transistor 246 and the high-side sensetransistor 250 to complete the high-side feedback loop. To ensurestability, the high-side feedback transistor 252 operates in triodingmode as a degeneration resistor.

The transistor 254 and the transistor 256 may be n-channel MOSFETs. Thecontrol terminals of the transistor 254 and the transistor 256 arecoupled to the control terminal of the transistor 238. The inputterminals of the transistor 254 and the transistor 256 are coupled tothe output terminal of the transistor 226 to control the voltage at thecontrol terminals of the low-side output transistor 248 and the low-sidesense transistor 260. The output terminal of the transistor 256 iscoupled to the input terminal of the high-side feedback transistor 252.The transistor 256 is included in some implementations of the class ABcontrol circuitry 210 to reduce the gain of the high-side feedback loop.Some implementations class AB control circuitry 210 may not include thetransistor 256. Current subtracted from the control terminal of thehigh-side output transistor 246 by operation of the high-side feedbacktransistor 252 is added to the control terminal of the low-side outputtransistor 248 to keep the overall gain of the class AB amplifier 100stable.

The transistor 258 is connected as a diode and included in someimplementations to reduce the impedance at the control terminal of thehigh-side feedback transistor 252, which improves the stability of thehigh-side feedback loop. The transistor 258 may be an n-channel MOSFET.The input terminal and the control terminal of the transistor 258 arecoupled to the output terminal of the high-side sense transistor 250.The output terminal of the transistor 258 is coupled to the referencevoltage source 278 via a resistor 276. In some implementations of theclass AB control circuitry 210, the current source 274 may be connectedin series with the resistor 276 to limit current in the outputtransistor 246 when load current is high.

Operation of the low-side feedback circuit is similar to that of thehigh-side feedback circuit. The low-side feedback circuit includes alow-side sense transistor low-side sense transistor 260, and a low-sidefeedback transistor 262. The low-side sense transistor 260 may be ann-channel MOSFET, and the low-side feedback transistor 262 may be ap-channel MOSFET. The low-side sense transistor 260, the low-sidefeedback transistor 262, and the transistor 226 of the folded cascodestage 204 form a feedback loop that controls the minimum current flowingin the low-side output transistor 248 and the low-side sense transistor260 to be the current provided by the current source 270. The low-sidesense transistor 260 is arranged to sense the current flowing in thelow-side output transistor 248. The control terminal of the low-sidesense transistor 260 is coupled to the control terminal of the low-sideoutput transistor 248, and the output terminal of the low-side sensetransistor 260 is coupled to the reference voltage source 278. The inputterminal of the low-side sense transistor 260 is coupled to the currentsource 270 and to the control terminal of the low-side feedbacktransistor 262. The current source 270 is coupled to the referencevoltage source 280.

The current flowing in the low-side sense transistor 260 is proportionalto the current flowing in the low-side output transistor 248.Accordingly, the voltage at the control terminal of the low-sidefeedback transistor 262 is inversely proportional to the current flowingin the low-side output transistor 248, and the voltage at the controlterminal of the low-side feedback transistor 262 increases as thecurrent flowing in the low-side output transistor 248 decreases. Theresistance of the low-side feedback transistor 262 increases withincreasing control voltage, which in-turn changes the ratio of currentflowing in the transistor 226 to the current flowing in the transistor264 and the transistor 266. The transistor 226 is coupled to the controlterminals of the low-side output transistor 248 and the low-side sensetransistor 260 to complete the low-side feedback loop. To ensurestability, the low-side feedback transistor 262 operates in triodingmode as a degeneration resistor.

The transistor 264 and the transistor 266 may be p-channel MOSFETs. Thecontrol terminals of the transistor 264 and the transistor 266 arecoupled to the control terminal of the transistor 226. The outputterminals of the transistor 264 and the transistor 266 are coupled tothe input terminal of the transistor 238 to control the voltage at thecontrol terminals of the high-side output transistor 246 and thehigh-side sense transistor 250. The input terminal of the transistor 266is coupled to the output terminal of the low-side feedback transistor262. The transistor 266 is included in some implementations to reducethe gain of the low-side feedback loop. Some implementations of theclass AB control circuitry 210 may not include the transistor 266.Current subtracted from the control terminal of the low-side outputtransistor 248 by operation of the low-side feedback transistor 262 isadded to the control terminal of the high-side output transistor 246 tokeep the overall gain of the class AB amplifier 100 stable.

The transistor 268 is connected as a diode and included in someimplementations to reduce the impedance at the control terminal of thelow-side feedback transistor 262, which improves the stability of thelow-side feedback loop. The transistor 268 may be a p-channel MOSFET.The output terminal and the control terminal of the transistor 268 arecoupled to the input terminal of the low-side sense transistor 260. Theinput terminal of the transistor 268 is coupled to the reference voltagesource 280 via a resistor 272. In some implementations of the class ABcontrol circuitry 210, the current source 270 may be connected in serieswith the resistor 272 to limit current in the output transistor 248 whenload current is high.

The class AB control circuitry, and the class AB amplifier 100, operateat power supply voltages as lows 1.2 volts. Because the class AB controlcircuitry 210 does not require the use low threshold voltagetransistors, the class AB control circuitry 210 can be fabricated usinga variety of semiconductor processes that do not support fabrication oflow threshold voltage transistors. Furthermore, because the class ABcontrol circuitry 210 does not require the use low threshold voltagetransistors the gain provided by the class AB amplifier 100 is higherthan class AB amplifiers that achieve low voltage operation using lowthreshold voltage transistors.

The above discussion is meant to be illustrative of the principles andvarious embodiments of the present invention. Numerous variations andmodifications will become apparent to those skilled in the art once theabove disclosure is fully appreciated. It is intended that the followingclaims be interpreted to embrace all such variations and modifications.

What is claimed is:
 1. An amplifier, comprising: an input stage; afolded cascode stage coupled to the input stage; and a class AB outputstage coupled to the folded cascode stage, the class AB output stagecomprising: a high-side output transistor; a low-side output transistor;and a high-side feedback circuit coupled to the high-side outputtransistor, the high-side feedback circuit comprising: a high-side sensetransistor comprising a control terminal, wherein the control terminalof the high-side sense transistor is coupled to a control terminal ofthe high-side output transistor; and a high-side feedback N-typefield-effect transistor (NFET), a gate of the high-side feedback NFETcoupled to an output of the high-side sense transistor and a source ofthe high-side feedback NFET coupled to the folded cascode stage; whereina first output of the folded cascode stage is coupled to the controlterminal of the high-side sense transistor and the control terminal ofthe high-side output transistor.
 2. The amplifier of claim 1, furthercomprising: a low-side feedback circuit coupled to the low-side outputtransistor, the low-side feedback circuit comprising: a low-side sensetransistor comprising a control terminal, wherein the control terminalof the low-side sense transistor is coupled to a control terminal of thelow-side output transistor; and a low-side feedback transistor coupledto an output of the low-side sense transistor and to the folded cascodestage; wherein a second output of the folded cascode stage is coupled tothe control terminal of the low-side sense transistor and the controlterminal of the low-side output transistor.
 3. A class AB amplifier,comprising: a folded cascode stage; and a class AB output stage coupledto the folded cascode stage, the class AB output stage comprising: ahigh-side output transistor having a control terminal, a first currentterminal and a second current terminal, wherein the first currentterminal is coupled to a supply voltage source; a low-side outputtransistor having a control terminal, a first current terminal and asecond current terminal, wherein the first current terminal is coupledto a reference voltage source and the second current terminal is coupledto the second current terminal of the high-side output transistor; ahigh-side feedback circuit coupled to the high-side output transistor,the high-side feedback circuit including: a high-side sense transistorhaving: a control terminal coupled to the control terminal of thehigh-side output transistor, a first current terminal coupled to thesupply voltage source, and a second current terminal; and a firstcurrent source having a first terminal coupled to the second currentterminal of the high-side sense transistor and a second terminal coupledto the reference voltage source; wherein a first output of the foldedcascode stage is coupled to the control terminal of the high-side sensetransistor and to the control terminal of the high-side outputtransistor; a low-side feedback circuit coupled to the low-side outputtransistor, the low-side feedback circuit including: a low-side sensetransistor having: a control terminal coupled to the control terminal ofthe low-side output transistor, a first current terminal coupled to thereference voltage source, and a second current terminal; and a secondcurrent source having a first terminal coupled to the second currentterminal of the low-side sense transistor and a second terminal coupledto the first terminal of the high-side sense transistor; wherein asecond output of the folded cascode stage is coupled to the controlterminal of the low-side sense transistor and to the control terminal ofthe low-side output transistor.
 4. The class AB amplifier of claim 3,further comprising a high-side feedback transistor having a controlterminal coupled to the second current terminal of the high-side sensetransistor and a current terminal coupled to the folded cascode stage.5. The class AB amplifier of claim 3, further comprising a low-sidefeedback transistor having a control terminal coupled to the secondcurrent terminal of the low-side sense transistor and a current terminalcoupled to the folded cascode stage.
 6. A class AB amplifier,comprising: a high-side output transistor having a control terminal, afirst current terminal and a second current terminal, wherein the firstcurrent terminal is coupled to a supply voltage source; a low-sideoutput transistor having a control terminal, a first current terminaland a second current terminal, wherein the first current terminal iscoupled to a reference voltage source and the second current terminal iscoupled to the second current terminal of the high-side outputtransistor; a folded cascode stage coupled to the high-side outputtransistor and the low-side output transistor; a high-side feedbackcircuit coupled to the high-side output transistor, the high-sidefeedback circuit including: a high-side sense transistor having: acontrol terminal coupled to the control terminal of the high-side outputtransistor, a first current terminal coupled to the supply voltagesource, and a second current terminal; and a high-side feedbacktransistor having a control terminal coupled to the second currentterminal of the high-side sense transistor and a current terminalcoupled to the folded cascode stage; wherein a first output of thefolded cascode stage is coupled to the control terminal of the high-sidesense transistor and to the control terminal of the high-side outputtransistor; and a low-side feedback circuit coupled to the low-sideoutput transistor, the low-side feedback circuit including: a low-sidesense transistor having: a control terminal coupled to the controlterminal of the low-side output transistor, a first current terminalcoupled to the reference voltage source, and a second current terminal;and a low-side feedback transistor having a control terminal coupled tothe second current terminal of the low-side sense transistor and acurrent terminal coupled to the folded cascode stage; wherein a secondoutput of the folded cascode stage is coupled to the control terminal ofthe low-side sense transistor and to the control terminal of thelow-side output transistor.